A semiconductor device such as an LED and a power module is provided with a structure in which a semiconductor element is bonded to a circuit layer formed from a conductive material.
In a large-power control power semiconductor element that is used to control wind power generation, an electric vehicle, a hybrid vehicle, and the like, the amount of heat generation is great. According to this, as a substrate on which the power semiconductor element is mounted, for example, a power module substrate including a ceramic substrate formed from aluminum nitride (AlN), alumina (Al2O3), and the like, and a circuit layer formed by bonding a metal plate with excellent conductivity on one surface of the ceramic substrate has been widely used in the related art. Furthermore, as the power module substrate, a power module substrate, in which a metal layer is formed on the other surface of the ceramic substrate, is also provided.
For example, a power module disclosed in Patent Document 1 includes a power module substrate in which a circuit layer and a metal layer which are constituted by Al are respectively formed on one surface and the other surface of a ceramic substrate, and a semiconductor element that is bonded onto the circuit layer through a solder material.
In addition, a heat sink is bonded to a lower side of the power module substrate to radiate heat, which is transferred from the semiconductor element to the power module substrate side, to an outer side through the heat sink.
However, as is the case with the power module described in Patent Document 1, in a case where the circuit layer and the metal layer are constituted by Al, an oxide film of Al is formed on a surface, and thus it is difficult to bond the semiconductor element or the heat sink onto the surface with the solder material.
Accordingly, for example, as disclosed in Patent Document 2, after a Ni plating film is formed on the surface of the circuit layer and the metal layer through electroless plating and the like, the semiconductor element or the heat sink is soldered.
Patent Document 3 suggests a technology of bonding the circuit layer and the semiconductor element, and bonding the metal layer and the heat sink, by using silver oxide paste, which contains silver oxide particles and a reducing agent composed of an organic, as an alternative of the solder material.
However, as described in Patent Document 2, in the power module substrate in which the Ni plating film is formed on the surface of circuit layer and the surface of the metal layer, during bonding of the semiconductor element and the heat sink, a surface of the Ni plating film deteriorates due to oxidation and the like, and thus there is a concern that bonding reliability of the semiconductor element and the heat sink which are bonded through the solder material deteriorates. In addition, in a Ni plating process, a masking process may be performed in order for a trouble such as electrolytic corrosion due to formation of the Ni plating in an unnecessary region not to occur. As described above, in a case of performing a plating process after performing the masking process, a great deal of labor is necessary in the step of forming the Ni plating film on the surface of the circuit layer and the surface of the metal layer, and thus there is a problem that the manufacturing cost of the power module greatly increases.
As described in Patent Document 3, in a case of bonding the circuit layer and the semiconductor element, and the metal layer and the heat sink, respectively, by using the silver oxide paste, bondability between Al and a sintered body of the silver oxide paste is poor, and thus it is necessary to form a Ag underlying layer on the surface of the circuit layer and the surface of the metal layer in advance.
Accordingly, Patent Document 4 suggests a power module in which the circuit layer and the metal layer are set to have a laminated structure of an Al layer and a Cu layer. In this case, the Cu layer is disposed on the surface of the circuit layer and the metal layer, it is possible to bond the semiconductor element and the heat sink by using the solder material in a satisfactory manner. In addition, deformation resistance of Cu is greater in comparison to Al. Accordingly, when a heat cycle is applied to the power module, it is possible to suppress great deformation of the surface of the circuit layer and the surface of the metal layer. As a result, occurrence of a crack in a solder layer is prevented, and thus it is possible to improve bonding reliability between the semiconductor element and the circuit layer, and the heat sink and the metal layer.
In the power module described in Patent Document 4, a bonded body, in which the Al layer and the Cu layer are bonded through a Ti layer, is used as the circuit layer and the metal layer. Here, a diffusion layer is formed between the Al layer and the Ti layer, and this diffusion layer includes an Al—Ti layer, an Al—Ti—Si layer, and an Al—Ti—Cu layer in this order from an Al layer side.